Image sensors based on semiconductor components work on the principle of converting photons into electron/hole pairs in silicon. More specifically, the charges created in the photosensitive regions are stored in the photodiode and are then read by an electronic system. This electronic system, which operates the photodiode, includes, especially when the photodiode is a fully depleted photodiode, a transfer transistor permitting transfer of the charges stored in the photodiode.
If no precautions are taken, under blooming conditions, the electrons that can no longer be stored in the photodiode can then diffuse into the semiconductor substrate where they disturb all the photodiodes of the image sensor. The visual result of this is an increasingly large white halo around the image.
To remedy this problem, one solution consists in defining the photodiode saturation level by the voltage applied to the gate of the transfer transistor. More specifically, this voltage is chosen so that the charges will be drained away towards the supply (via controlled conduction of a resetting transistor) before they diffuse into the substrate and degrade the signal received by the neighbouring photodiodes.
However, this voltage must be adjusted by a specific supply and must take into account the dispersions associated with the technology. Furthermore, these dispersions result in a reduction in the storage dynamic range of the photodiode.